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Date: Fri, 26 Jun 2020 13:29:31 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Masahiro Yamada <masahiroy@...nel.org>,
Will Deacon <will@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Kees Cook <keescook@...omium.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
clang-built-linux@...glegroups.com,
kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
x86@...nel.org, Josh Poimboeuf <jpoimboe@...hat.com>,
mhelsley@...are.com
Subject: Re: [RFC][PATCH] objtool,x86_64: Replace recordmcount with objtool
On Thu, Jun 25, 2020 at 03:40:42PM -0700, Sami Tolvanen wrote:
> > Not boot tested, but it generates the required sections and they look
> > more or less as expected, ymmv.
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index a291823f3f26..189575c12434 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -174,7 +174,6 @@ config X86
> > select HAVE_EXIT_THREAD
> > select HAVE_FAST_GUP
> > select HAVE_FENTRY if X86_64 || DYNAMIC_FTRACE
> > - select HAVE_FTRACE_MCOUNT_RECORD
> > select HAVE_FUNCTION_GRAPH_TRACER
> > select HAVE_FUNCTION_TRACER
> > select HAVE_GCC_PLUGINS
>
> This breaks DYNAMIC_FTRACE according to kernel/trace/ftrace.c:
>
> #ifndef CONFIG_FTRACE_MCOUNT_RECORD
> # error Dynamic ftrace depends on MCOUNT_RECORD
> #endif
>
> And the build errors after that seem to confirm this. It looks like we might
> need another flag to skip recordmcount.
Hurm, Steve, how you want to do that?
> Anyway, since objtool is run before recordmcount, I just left this unchanged
> for testing and ignored the recordmcount warnings about __mcount_loc already
> existing. Something is a bit off still though, I see this at boot:
>
> ------------[ ftrace bug ]------------
> ftrace failed to modify
> [<ffffffff81000660>] __tracepoint_iter_initcall_level+0x0/0x40
> actual: 0f:1f:44:00:00
> Initializing ftrace call sites
> ftrace record flags: 0
> (0)
> expected tramp: ffffffff81056500
> ------------[ cut here ]------------
>
> Otherwise, this looks pretty good.
Ha! it is trying to convert the "CALL __fentry__" into a NOP and not
finding the CALL -- because objtool already made it a NOP...
Weird, I thought recordmcount would also write NOPs, it certainly has
code for that. I suppose we can use CC_USING_NOP_MCOUNT to avoid those,
but I'd rather Steve explain this before I wreck things further.
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